This invention pertains generally to the field of pressure sensors and particularly to micromachined pressure sensors.
Conventional pressure sensors have typically been large, discrete devices formed using a large diaphragm, usually of metal, as the pressure barrier. To minimize the fabrication cost and the size of components, and to reduce the size of the pressure sensor structure to a size comparable to that of integrated circuits to which the sensors are interfaced, significant efforts have been made to produce micromachined pressure sensors on silicon substrates. For example, pressure sensors have been formed by selective etching of a silicon substrate until the etch is terminated by a highly doped layer to form a thin membrane, and by thereafter bonding another member over the membrane to define a cavity, and by forming polysilicon membranes on the surface of a single crystal silicon substrate.
The diaphragm of a pressure sensor deflects in response to the differential pressure across it in a manner which is related to the differential pressure. The deflection of the diaphragm can be measured in various ways, including the use of piezoresistive elements formed on the diaphragm which change in resistance as a function of the strain in the diaphragm, and by capacitive sensing in which a surface of the diaphragm forms one of the plates of a capacitor and the other plate is formed on the substrate beneath the diaphragm. While capacitive sensing in this manner has advantages over piezoresistive sensing and is commonly used, both for conventional large, discrete pressure sensors and for micromachined sensors, microfabrication of such devices is typically complicated. In particular, where the bottom plate or electrode of the capacitor is sealed in a cavity under the diaphragm, transferring a signal from the plate to the exterior of the cavity, where the signal can be transmitted to interface electronics, presents a formidable manufacturing challenge. In general, several additional microfabrication steps are required to successfully extend a lead from the sealed cavity to the exterior, with the overall process becoming quite complicated, requiring as many as ten masking steps. See, e.g., A. V. Chavan and K. D. Wise, xe2x80x9cA Batch-Processed Vacuum-Sealed Capacitive Pressure Sensor,xe2x80x9d Proc. Int""l. Conf. On Solid-State Sensors and Actuators, Chicago, Ill., June 1997, pp. 1449-1452; A. V. Chavan and K. D. Wise, xe2x80x9cA Multi-Lead Vacuum-Sealed Capacitive Pressure Sensor,xe2x80x9d Proc. Solid-State Sensor and Actuator Workshop, Hilton Head Island, S.C., June 1998, pp. 212-215. Some approaches accomplish hermetic sealing of the sensor cavity with techniques that are not generally compatible with lithography-based batch-fabrication techniques. Y. Wang and M. Esashi, xe2x80x9cA Novel Electrostatic Servo Capacitive Vacuum Sensor,xe2x80x9d Proc. Int""l. Conf. On Solid-State Sensors and Actuators, Chicago, Ill., June, 1997, pp. 1457-1460.
A micromachined pressure sensor in accordance with the invention can be formed utilizing a minimum number of masking and processing steps while avoiding the need to extend an electrical lead through a wall of the sealed cavity of the pressure sensor. Changes in pressure result in deflection of structure having capacitive plates formed thereon which are external to the sealed cavity, so that electrical leads can be readily connected to the plates formed on such structures. The capacitive pressure sensors can be formed in a manner which is compatible with conventional lithographic processing of silicon substrates as carried out in standard integrated circuit production, allowing such pressure sensors to be interfaced with integrated circuits. The deformable structures of the sensor may be formed of conventional materials used in microelectronic processing, such as crystalline silicon, in an efficient and economical production process.
A pressure sensor in accordance with the invention includes a substrate, a base secured to the substrate and a diaphragm secured to the base to define a sealed cavity between the base, substrate and diaphragm. A first electrode is formed on at least one of the diaphragm or base to deflect therewith and a second electrode is formed adjacent thereto. An electrode formed, for example, on the substrate may comprise the second electrode and form one of the plates of a capacitor. A skirt extending outwardly from the base above the substrate may form the first electrode as the other plate of the capacitor, with at least one of the diaphragm or the base deforming and deflecting with changes in ambient pressure to deflect the skirt toward or away from the other electrode to change the effective capacitance therebetween in a manner which is related to the changes in pressure.
The base may be formed as a hollow cylinder and the diaphragm and skirt may be formed integrally together as a flat, circular plate which is secured to the top of the base (e.g., by being integrally formed therewith) to provide a sealed cavity under the diaphragm and with the skirt extending outwardly from the periphery of the base over an electrode formed on the substrate. The skirt itself may form one of the plates of the variable capacitor by, for example, being formed of heavily doped and electrically conductive silicon, with the other electrode plate of the capacitor deposited as a metal film on the substrate under the skirt. Alternatively, two separate electrodes forming the plates of the capacitor may be formed underneath the skirt and being coupled to each other through the skirt so that deflections of the skirt change the relative capacitance between the two separated plates. The skirt may also be formed to extend from the base at positions intermediate the top and bottom of the base rather than extending from the diaphragm, and may be formed to have various modified cantilever structures which position a section of the skirt more closely adjacent to the underlying capacitor plate on the substrate while maintaining the full height of the base. The base may also be formed with alternative structures in multiple parts with the skirt extending between the deformable base structures, and the skirt itself may be integrally formed as a wall of the base with the skirt deflecting in response to changes in pressure within the sealed cavity to change the relative position of the sidewall of the base, functioning as the skirt, with respect to an adjacent electrode plate. Because the skirt may be formed to deflect away from the adjacent electrode with increasing ambient pressure, the pressure sensors of the present invention are well suited to feedback control in which a voltage is applied across the plates of the capacitor at an appropriate level to deflect the skirt back toward a reference position.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.